In the sound-induced flash illusion, when two beeps are presented simultaneously with a flash, a second illusory flash is perceived (Shams, Kamitani & Shimojo, 2000). Even though this phenomenon has been examined by a number of authors (e.g., Andersen, Tiippana & Sams, 2004; Berger, Martelli & Pelli, 2003; McCormick & Mamassian, 2008; Shams, Kamitani & Shimojo, 2002), little is known about the exact parameters driving the illusion. A recent study found that the effect is not as robust as it was previously thought, and there is a large disparity in the occurrence of the illusion from a subject to another (Mishra, Martinez, Sejnowski & Hillyard, 2007). A number of reasons might explain this variable occurrence of the illusion but one of them is that the parameters used to generate the illusion are suboptimal. It has been shown that timing is an important aspect of multisensory illusory effects, such as in the McGurk (Munhall, Gribble, Sacco & Ward, 1996) and the ventriloquist effects (Slutsky & Recanzone, 2001). Here, we investigated the temporal constraints of the sound-induced flash illusion. More specifically, we studied the effect of the variance in the timing of the flashes and the beeps using a classification image technique. Results will be given a Bayesian interpretation.